Intercrystalline pores

Different ways have been proposed to prepare zeolite membranes. A layer of a zeolite structure can be synthesized on a porous alumina or Vycor glass support [27, 28]. Another way is to allow zeolite crystals to grow on a support and then to plug the intercrystalline pores with a dense matrix [29], However, these two ways often lead to defects which strongly decrease the performance of the resulting membrane. A different approach consists in the direct synthesis of a thin (but fragile) unsupported monolithic zeolite membrane [30]. Recent papers have reported on the preparation of zeolite composite membranes by hydrothermal synthesis of a zeolite structure in (or on) a porous substrate [31-34]. These membranes can act as molecular sieve separators (Fig. 2), suggesting that dcfcct-frcc materials can be prepared in this way. The control of the thickness of the separative layer seems to be the key for the future of zeolite membranes. 

Samples used in this study, their formation, petrographic, petrophysical and mineralogical characteristics. C crystal carbonate M-W mudstone, wackestone P-G packstone-grainstone Vac vugs iX intercrystalline pores iM intramatrix pores iG intragranular pores IG intergranular pores K karsts Fr fractures F formation factor m cementation factor n saturation exponent. 

Figure 30 compares the long-range diffusivity of n-butane in a loose bed of NaX zeolite crystallites with that of the same sample after compaction under a pressure of 2.5 MPa. It is found that long-range diffusion in the compacted material is reduced by a factor on the order of 3, which may be attributed to a reduction of both (diminution of the intercrystalline void volume) and (decrease of the intercrystalline pore diameters and hence of the effective mean free path). This experimental result confirms that the contribution of intra-... 

It is known that zeolite membranes essentially contain intercrystalline non-zeolitic pores (defects). This irregular nature of zeolite membranes with intercrystalline pores adds to the complexity of the transport process in addition to the contribution of a support layer to the permeation resistance. For zeolite membranes, selectivity similar to that expected for Knudsen flow generally indicates the presence of intercrystalline pores. Separation based primarily on adsorption differences, which is generally true in the separation of liquid mixtures by pervaporation, may have tolerance to the intercrystalline pores. However, in order to obtain high perm-selectivity, the zeolite membranes must have negligible amounts of intercrystalline pores and pinholes of larger than 2nm so as to reduce the gas flux from these defects [3]. 

Zeolite membranes have long been recognized to have great potential for gas separations. However, considerable challenges, such as elimination of intercrystalline pores and reduction of pore size to increase the hydrogen selectivity, remain to be resolved. 

Replacement of calcite by dolomite increases porosity by 0.13 (or 13%) and creates important reservoir space, and the new intercrystalline pores improve the connectivity of the pore network. Peters (2012) noted Good porosity in carbonate reservoirs is usually due to dolomitization [...]. Due to their large surface area, mud-sized grains are more easily dolomitized than sand-sized grains. Thus, the best carbonate reservoirs may have the lowest primary porosity. Dolomitization also creates planar crystal surfaces and harder crystal structures. Thus, dolomite retains more of their porosity during compaction than limestone (compare Fig. 2.10). 

The important property of ZSM-5 and similar zeolites is the intercrystalline catalyst sites, which allow one type of reactant molecule to diffuse, while denying diffusion to others. This property, which is based on the shape and size of the reactant molecules as well as the pore sizes of the catalyst, is called shape selectivity. Chen and Garwood document investigations regarding the various aspects of ZSM-5 shape selectivity in relation to its intercrystalline and pore structure. 

When the oxide is formed by anodizing in acid solutions and the sample is then left to rest at the OCP, some dissolution can occur. This process has been studied by a numbers of authors,70-75 especially in relation to porous oxides [cf. Section 111(4)]. It was found that pore walls are attacked, so that they are widened and tapered to a trumpet-like shape.70 71 Finally, the pore skeleton collapses and dissolves, at the outer oxide region. The outer regions of the oxide body dissolve at higher rates than the inner ones.9,19 The same is true for dissolution of other anodic oxides of valve metals.76 This thickness dependence is interpreted in terms of a depth-dependent vacancy concentration in the oxide75 or by acid permeation through cell walls by intercrystalline diffusion, disaggregating the microcrystallites of y-alumina.4... 

Adsorption details of calcined DD3R crystals are listed in Table 8 [36]. As can be seen from Table 8, a complete separation between linear and branched alkanes can be achieved. Isobutane is excluded from the pore structure of DD3R. The very small adsorption observed is ascribed to external surface or intercrystalline sorption. 

In general, the properties and separation abilities of the resulting membranes depend on the synthesis procedure. The amount of zeolitic material, support composition, penetration and adhesion to the support, orientation of the zeolite crystals, the density and distribution of nonzeolitic pores (i.e., intercrystalline voids), crystal boundaries, and the thickness of the zeolite layer are the main variables which affect the quality of the obtained membrane. 

Posttreatment processes have been used to improve the quahty of the resulting membranes, such as ion exchange (to provide catalytic properties or change them between hydrophobic and hydrophilic surfaces), liquid or vapor sililation, coke deposition, CVD (chemical vapor deposition), and ALCVD (atomic layer chemical vapor deposition). These techniques are used to reduce the intercrystalline gaps and the pore-mouth size, modify the acid properties of the modified membranes, and remove amorphous material. Some of these modifications have demonstrated very high separation selectivities for the resulting membranes however, in many cases, they are of limited practical application due to the relatively low fluxes obtained. 

It is well established that the smallest crystals are the most effective as catalysts as long as the catalytic reaction proceeds in the intercrystalline void volume [1,8]. Increased crystal size will result in an increase in pore length and thus in the Thiele modulus. This will result in a reduced effectiveness factor, viz. a reduced actual rate of reaction. 

Dullien FAL (1979) Porous Media Fluid Transport and Pore Stracture. Academic Press, New York Eiler JM, Baumgartner LP, Valley JW (1992) Intercrystalline stable isotope diffusion a fast grain bonndary model. Contrib Mineral Petrol 112 543-557... 

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